We report high-spatial-resolution measurements of the thermal boundary layer (BL) properties in turbulent thermal convection. The experiment was made near the lower conducting plate of a water-filled rectangular convection cell of height 0.76 m, with a Prandtl number
$\mathit{Pr}= 4. 3$
and over the Rayleigh-number range
$2\times 1{0}^{10} \lt \mathit{Ra}\lt 7\times 1{0}^{11} $
. Time series of the local temperature at various vertical distance
$z$
from the plate were measured. Statistical properties of the profiles of the temperature, i.e. the mean temperature
$\langle T\rangle $
, fluctuating temperature root mean square (r.m.s.)
${\sigma }_{T} $
, temperature skewness
${S}_{T} $
, and flatness
${F}_{T} $
, and those of the temperature time derivative, i.e. the r.m.s.
${ \sigma }_{T}^{\prime } $
, skewness
${ S}_{T}^{\prime } $
and flatness
${ F}_{T}^{\prime } $
of the derivative, are studied. It is found that most of these quantities exhibit some degree of invariability with
$\mathit{Ra}$
, especially for the regime inside the thermal BL. When comparing with the mean temperature profiles, the profiles of the second moment of temperature seem to possess a higher level of universality. It is shown that the distance
${\delta }_{\sigma } $
from the plate to the maximal temperature r.m.s. position provides a natural length scale for the characterization of the thermal BL, as the statistical properties of the temperature field, such as its r.m.s., skewness and flatness, are all sharply different below and above this length scale, i.e. below
${\delta }_{\sigma } $
,
${\sigma }_{T} $
increases linearly with the vertical distance
$z$
from the plate and
${S}_{T} $
is close to zero and
${F}_{T} $
is close to three and both quantities remains nearly constant, whereas above
${\delta }_{\sigma } $
the decay of
${\sigma }_{T} $
obeys a logarithmic behaviour and
${S}_{T} $
and
${F}_{T} $
both exhibit a hill-like structure. It is also found that near the plate
$\langle T\rangle $
,
${\sigma }_{T} $
and
${ \sigma }_{T}^{\prime } $
all increase linearly with
$z$
. Our observations further reveal that such linear dependence occurs within a self-similar region of the thermal BL, where the temperature probability density functions can be scaled onto a single distribution that differs slightly from the Gaussian distribution. The
$\mathit{Ra}$
-dependencies of various thermal BL properties are also studied and our results yield
${\delta }_{th} / H= (6. 85\pm 0. 70){\mathit{Ra}}^{- 0. 33\pm 0. 03} $
,
${\delta }_{\sigma } / H= (2. 86\pm 0. 30){\mathit{Ra}}^{- 0. 31\pm 0. 03} $
and
${ \delta }_{\sigma }^{\prime } / H= (25\pm 3){\mathit{Ra}}^{- 0. 38\pm 0. 05} $
, where
$H$
is the height of the cell,
${\delta }_{th} $
and
${ \delta }_{\sigma }^{\prime } $
are the BL thicknesses determined respectively from the profiles of
$\langle T\rangle $
and
${ \sigma }_{T}^{\prime } $
.